Semiconductor injection lasers are greatly suited to high data rate fiber optic transmission systems since they can be modulated in the GHz range and have a narrow, well-defined output bandwidth.
Two difficulties of operating semiconductor lasers at high modulation rates are that (i) transient relaxation oscillations are generated when the laser is first switched on and (ii) in certain circumstances which have not yet been fully determined, semiconductor lasers exhibit sustained pulsations within a range of bias conditions. In the former case, the oscillation frequency is of the order of 1 GHz and the laser reaches a steady state within a few nanoseconds. In the latter case, the pulsations, which have a frequency of from 0.2 to 2 GHz, are believed to derive from crystalline defects including those defects which develop during laser operation. This explains why some lasers do not exhibit sustained pulsations at all, while others are subject to pulsations only as the devices age.
Chinone, et al (Applied Physics Letters, Vol. 33, Page 12, 15 December 1978) show that relaxation oscillations and sustained pulsations in a semiconductor laser output can be suppressed by feeding back into the laser resonant cavity light reflected from an external mirror located close to the laser output facet. Chinone et al demonstrate an arrangement for achieving oscillation suppression which uses a laser, a converging lens separate from the laser and a plane mirror separate from the lens. Light from the laser is rendered parallel at the converging lens and is transmitted through an air gap of the order of 1 cm. in length before being reflected back through the lens to the laser.
Obviously, although the method and device disclosed by Chinone and later, in as modified form, by Kobayishi (U.S. Pat. No. 4,079,339) may be quite satisfactory to the extent to which self-pulsations are quenched, it would be preferable if quenching took place within the semiconductor chip itself without recourse to an external reflector and light collimating means, the latter being both bulky and tedious to adjust.